Envelope detector

Abstract

Envelope detector comprising a differential amplifier (A), a full-wave rectifier circuit and a smoothing filter (AF), which differential amplifier (A) converts an input voltage into a pair of current signals in phase opposition and which full-wave rectifier circuit comprises a pair of half-wave rectifiers (HWR1, HWR2) for half-wave rectifying the said pair of current signals, coupled to a first combining stage (S) for combining the output signals of the two half-wave rectifiers (HWR1, HWR2) with like polarity, which first combining stage (S) supplies a full-wave rectified signal. In order to reduce d.c. offsets, the detector comprises a frequency-dependent negative feedback loop which comprises a second combining stage (D) coupled to the outputs (R o1 , R o2 ) of the pair of half-wave rectifiers (HWR1, HWR2) for combining the output signals of the pair of half-wave rectifiers (HWR1, HWR2) with opposite polarity, said second combining stage (D) being negatively fed back to the input (I c ) of the differential amplifier (A) via a low-pass filter (LPF) for a selection of the d.c. component.

What is claimed is: 
     
       1. An envelope detector comprising a differential amplifier, a full-wave rectifier circuit and a smoothing filter, said differential amplifier converting an input voltage into a pair of current signals in phase opposition, said full-wave rectifier circuit comprising a pair of half-wave rectifiers for half-wave rectifying the said pair of current signals and furnishing respective half-wave rectifier output signals corresponding to the so-rectified current signals, and a first combining stage coupled to said full-wave rectifier circuit for combining said half-wave rectifier output signals with like polarity, characterized by a frequency-dependent negative feedback loop which has a second combining stage coupled to the outputs of the pair of half-wave rectifiers for combining said half-wave rectifier output signals with opposite polarity, said second combining stage being negatively fed back to the differential amplifier via a low-pass filter for a selection of the d.c. component. 
     
     
       2. An envelope detector as claimed in claim 1, characterized in that the pair of half-wave rectifiers is constituted by a diode pair which is arranged in the direction of conductivity between a pair of current outputs of the differential amplifier and ground, and whose diodes are fed with mutually equal diode bias currents. 
     
     
       3. An envelope detector as claimed in claim 1, characterized in that the differential amplifier comprises a transistor pair whose bases are coupled to an input terminal pair of the differential amplifier, whose emitters are connected via first and second emitter resistors, respectively, to a common emitter current source and whose collectors are coupled via first and second collector current sources, respectively, to a power supply voltage and to the pair of half-wave rectifiers, said current sources being coupled via a current mirror arrangement to a common reference current, said current mirror arrangement comprising a current bias circuit for biasing the current of the emitter current source at the sum value of the currents of the said two collector current sources. 
     
     
       4. An envelope detector as claimed in claim 2, characterized by a transistor pair whose emitters are coupled to the diodes of the said diode pair, whose bases are coupled to a common base bias voltage and whose collectors are coupled via the smoothing filter to both the first and the second combining stage, said smoothing filter being provided with a capacitance arranged between the two collectors and a pair of collector resistors arranged in the collector leads of the transistor pair.